1. Field of the Invention
This invention relates to a method and apparatus of electrostatic recording and in particular to a transfer type electrostatic recording method and apparatus.
2. Description of the Prior Art
A multi-stylus type electrostatic recording system is schematically shown in FIG. 1. As shown, a plurality of recording styluses 2ij (i=1.about.n, j=1.about.m) are arranged in the form of a single array extending in the direction perpendicular to the recording medium advancing direction A. These recording styluses 2ij are grouped into blocks, i.e., n blocks in the example shown, and each block has the same number of recording styluses, i.e., m styluses in each block in the example shown. Corresponding recording styluses in respective blocks are commonly connected and they are also connected to a driving circuit B to which block-by-block information to be recorded is supplied. For each block of recording styluses is provided a pair of auxiliary electrodes 3, which are commonly connected. A switch S is connected between a voltage source (not shown) and the corresponding pair of auxiliary electrodes 3.
In operation, a series of bit information is stored into the driving circuit B, which is then applied to the corresponding recording styluses, and, at the same time, the switch S.sub.1 is turned on to apply a predetermined voltage to the auxiliary electrodes 3.sub.1 of the first block. Then, another series of bit information is stored in the driving circuit B, which is then applied to the corresponding recording styluses, and, at the same time, the switch S.sub.2 is turned on to apply a predetermined voltage to the auxiliary electrodes 3.sub.2 of the second block. In this manner, each block is sequentially operated from left to right in FIG. 1. Such a block formation is advantageous because the driving circuit B may be commonly used and the wiring is relatively simplified.
As shown in FIG. 2, the recording stylus 2 is supported by a recording unit 6 as electrically isolated therefrom, and a recording signal V.sub.sty is applied to the stylus 2. The auxiliary electrodes 3 are also supported by the recording unit 6 as electrically isolated therefrom, and a block signal V.sub.seg is applied thereto. The recording medium 1 includes a dielectric layer 11 which is formed on a conductive layer 12, which, in turn, is formed on a base layer 13, and the conductive layer 12 is grounded.
In the above-described electrostatic recording system, since the auxiliary electrodes 3 are provided and a predetermined voltage V.sub.seg of the polarity opposite to that of the recording signal V.sub.sty is applied thereto, the level of the recording signal V.sub.sty may be relatively low thereby allowing to use a relatively low voltage driver. When the above-described structure is applied to the transfer type recording system, in which a toner image formed on the recording medium 1 is transferred to plain paper thereby allowing to use the recording medium repetitively, residual charges remaining on the recording medium 1 after transfer must be removed. Under the circumstances, since it is extremely difficult to completely remove such residual charges to neutralize the surface potential of the recording medium 1, it is typically so structured that the recording medium 1 is first uniformly charged by a corona discharging device to make the surface potential uniform and then the charges thus deposited on the recording medium 1 are selectively removed by the discharge between the selected styluses 2 and the auxiliary electrodes 3 to form an electrostatic latent image, which is then developed to be converted into a toner image.
In the electrostatic recording system as described above, denoting the surface potential of the recording medium 1 when uniformly charged to be V.sub.S, the negative voltage to be applied to the styluses 2 at the time of recording to be V.sub.sty, and the positive voltage to be applied to the auxiliary electrodes 3 at the time of recording to be V.sub.seg, then the voltage existing between the selected styluses 2 and the recording medium 1 at the time of recording may be substantially given by the following equation. ##EQU1## where g is the gap distance between the stylus 2 and the recording medium 1, d.sub.m is the thickness of the dielectric layer 11, and .epsilon..sub.m is the dielectric constant of the dielectric layer 11. The relationship between V.sub.S, V.sub.seg and V.sub.sty is shown in FIG. 3.
In the above equation (1), since V.sub.S is positive and V.sub.sty is negative, V.sub.gap is simply an algebraic sum of these two values. Thus, if the value of V.sub.gap exceeds the discharge on-set voltage, then discharge will also take place between the selected styluses 2 and the recording medium 1 even in the unselected blocks, thereby forming a phantom image. For example, in the recording system shown in FIG. 1, even if only the switch S.sub.1 is turned on to apply V.sub.seg to the auxiliary electrodes 3.sub.1 of the leftmost block, the signal V.sub.sty is equally applied to the corresponding styluses, e.g., 2.sub.11, 2.sub.21, 2.sub.31, . . . and 2.sub.n1, in all of the blocks; consequently, writing will be carried out not only by the stylus 2.sub.11, which is wanted, but also by the corresponding styluses 2.sub.21 through 2.sub.n1, which are not wanted. It should also be noted that the level of V.sub.S may charge depending upon the condition, e.g., moisture content, of the recording medium 1.